1. Field of the Invention
The present invention relates to a recording apparatus for recording an image with dots on a recording medium.
2. Description of the Prior Art
Conventionally, when there is a need to shorten the transfer time of image signals during image formation in facsimiles and the like, the pitch of the scanning lines is increased. For this reason, the number of picture elements is small, the density is low and the quality of the image obtained is not satisfactory.
In order to improve the density, the same image signals are sometimes written in two lines. However, this does not shorten the recording time.
In the method for radiating a photosensitive body with a light beam and for visualizing a portion of the photosensitive body which is not radiated with the light beam (the unexposed part), "narrowing of the image" (narrowing of the image in the main scanning direction of the beam) occurs more than in the case of the method for visualizing a portion of the photosensitive body which is radiated with the light beam (the exposed part). This will be described in the further detail, with reference to a letter "T". In the method for visualizing the part radiated with the light beam, a letter "T" is formed as shown in FIG. 1, which has widths 101 and 102 in the main scanning direction. In contrast to this, in the method for visualizing the unexposed part, a letter "T" is formed as shown in FIG. 2, which has widths 201 and 202 in the scanning direction which are smaller than widths 101 and 102. This is called "narrowing of the image". Due to this phenomenon, the quality of the recorded letter "T" it not satisfactory in the method for visualizing the unexposed part. The reason for this phenomenon will be described with reference to FIG. 3A. In the method for visualizing the exposed part, in order to draw a line segment, a scanning spot at location 312 is formed on the recording medium at a leading edge 310 of an image signal 300. The scanning spot moves in a scanning direction 314 while the image signal 310 is on. The scanning spot at location 313 goes off at a trailing edge 311 of the image signal 300. In an exposure distribution 302 at a central line 315 of the scanning spot of the line segment, the light exposure increases from an end 316 to an end 318 of the scanning spot at location 312, remains constant for a time thereafter, and decreases from an end 319 to an end 321 of the scanning spot at location 313. The light exposure decreases to 1/2 the peak value at centers 317 and 320 of the scanning spot at locations 312 and 313. In the method for visualizing the exposed part, the part exposed at a level above a visualizing level 322 in the exposure distribution 302 is visualized while the part exposed at a level below the visualizing level 322 is not visualized. Therefore, the width of the visualized image becomes 323. The width 323 corresponds to the widths 101 and 102. In the method for visualizing the unexposed part, an image is formed with an image signal 304 which is an inverted signal of the image signal 300. The scanning spot at location 352 is on until a trailing edge 350 of the image signal 304 is generated and then the scanning spot goes off. At a leading edge 351 of the image signal 304, a scanning spot at location 353 is formed which moves in a scanning direction 354. In an exposure distribution 306 at a central line 355 of the scanning spot of the line segment, the light exposure decreases from an end 356 to and end 358 of the scanning spot 352 and then becomes zero. The exposure distribution 306 remains zero thereafter, increases from an end 359 to an end 361 of the scanning spot at location 353, and then remains constant again. The light exposure decreases to 1/2 the peak value at centers 357 and 360 of the scanning spot at locations 352 and 353. If a visualizing level in the method for visualizing the unexposed part is 362, the exposed and unexposed parts which are at levels above the level 362 are visualized and the remaining parts are not visualized. Therefore, the width of the visualized image becomes 363. This width corresponds to widths 201 and 202 in FIG. 2.
As has been described above, in the method for the unexposed part, the visualized width becomes 363 shown in FIG. 3B (or 201 and 202 in FIG. 2). This visualized part is more narrow than the visualized width 323 shown in FIG. 3A (or 101 or 102 in FIG. 1) produced according to the method for visualizing the exposed part. Therefore, the letter "T" as shown in FIG. 1 is recorded as a letter "T" as shown in FIG. 2. When such narrowing occurs, the quality of the image is thus degraded.
Narrowing does not present a big problem if a thick line is to be recorded since narrowing only occurs at the edges of the image in this case. However, in the case of a thin line, narrowing becomes noticeable. In order to solve this problem, Japanese Patent Application No. 8,112/81 proposes a method according to which the pulse width of the electric signal is increased in the main scanning direction so as to compensate for narrowing of the image. However, this prior art method does not provide a good solution to the problem of narrowing in the subscanning direction.